With transmission of data over a digital communication network, such as an asynchronous transfer mode or ATM network, problems arise when multiple sources send data cells or packets at widely varying rates through a switch node or link of the network at an aggregated rate which taxes the ability of the switch node or link to handle the data. Congestion occurs at the node of a network when data arrives at the node at a rate exceeding the rate at which the node can process and forward the data to other nodes. The excess data then accumulates in buffer storage at the node, which fills at a rate which is the difference between the arrival rate and the processing and forwarding rate. If the congestion persists for a long period of time, the buffer storage will be filled to maximum capacity and any additional data must be discarded.
In an effort to minimize such data loss, different types of flow control systems have been proposed. One such system is a rate-based system in which the rate at which the data is permitted to enter the network is adjusted via a feedback signal from a network reflecting the congestion of the network. This congestion of the network is typically denoted by an explicit forward congestion indication bit or an explicit rate in the form of resource management cells fed back to the source node.
Another system for controlling data transmission employs a credit-based control approach which provides lossless transmission of data cells. Credits are generated starting at a destination node to reflect its ability to receive data. In an end-to-end implementation, this credit is transmitted back to the next upstream node where the credit is interpreted and modified based on that node's ability to receive data. The process continues through each intermediate node back to the source, where the credit at the source reflects all intermediate credits as well as the one from the destination. Typically, the credits reflect the unused buffer space at each node. The source then interprets the credit as an indication of the amount of data that it can transmit into the network without any data loss due to congestion or buffer overflow. Note that data rate is not controlled, but the number of cells transmitted is controlled.
A variation on the end-to-end credit-based approach is a link-to-link implementation wherein adjacent nodes in a switch network, for example, interact to control the flow of data from one node, a sender, to another node, the receiver. The sender supplies data segments for forwarding to the receiver, and the receiver has a finite data receive buffer into which received data segments from the sender are placed. The emptying of the data receive buffer is controlled by a buffer read signal from a downstream entity. In an ideal uncongested communication fabric, each segment of data could be read from the data receive buffer the cycle after it is written therein from the sender. In such a case, the data receive buffer would never contain more than one data segment. When congestion causes the downstream entity to slow its rate of buffer reads below one per cycle, data segments will accumulate in the receive buffer. This reduces the space available for storing future data segments from the sender.
At the time the link is established, the sender is allocated a number of credits. Each credit represents permission to transmit one data segment over the data link to the receiver. Besides the data link, a separate credit link is employed by the receiver to provide the sender with additional credits. Because the credit link is separate from the data link, the transfer of credits has no effect on the data bandwidth. The sender increments a credit counter upon receipt of a credit and decrements the credit counter when a data segment is placed on the data link for transmission to the receiver.
The flow control method described above works perfectly in an error free link. The sender is never starved for credits unnecessarily and the receiver buffer is never overrun. However, single bit errors in either the data or credit link may introduce errors in the flow control mechanism which may result in performance degradation or overrun of the receive buffer. If the receive buffer is overrun, data segments will be lost. Conventionally, any error detected on the data from the sender to the receiver causes the link to stop running (fault) and be reset to a known state. Similarly, in prior systems if the credits flowing from the receiver to the sender have invalid code points, and if any of these are detected, the link would fault and be reset to a known state. Thus, the conventional error correction approach is not tolerant of occasional faults in the network, even if the faults comprise errors in credits transmitted from the receiver back to the sender.
Since prior credit-based flow control mechanisms are fault intolerant, there is a need for a tolerant credit-based flow control checking scheme and accompanying correction mechanism to maintain individual links in a system operating optimally.